Of the many conversion processes known in the refining art, hydrocracking has assumed a greater importance over the years since it offers the refiner product flexibility combined with product quality.
There has been considerable effort devoted to the development of hydrocracking catalysts which combine high cracking activity with a low tendency to overcrack towards light products and, in particular, to the less valuable C1-C3 and C4 gaseous by-products. Kerosine and gas oil (the fractions boiling at a temperature in the range of from 150 to 370° C.; also commonly termed middle distillates) are often the desired products of a hydrocracking process.
Such hydrocracking catalysts are typically based on a shaped support prepared from a single, active cracking component such as an aluminosilicate, especially a Y zeolite component, comulled and extruded with a refractory oxide binder, with subsequent impregnation of hydrogenation metals.
Alternative catalyst forms have been proposed for use in the hydroprocessing of, for example, refinery streams. One such group of catalysts are termed ‘bulk catalysts’. Such catalysts are formed from metal compounds only, usually by co-precipitation techniques, and have no need for a catalyst carrier or support; see for example WO 00/42119, U.S. Pat. No. 6,162,350 and WO 00/41810. These publications disclose bulk Group VIII and Group VIb metal catalysts and their preparation and use. U.S. Pat. No. 6,162,350 discloses that such catalysts may contain one or more of each metal type, and examples show NiMo, NiW and the most preferred NiMoW bulk catalysts A binder if present is preferably added after the preparation of the bulk metal composition and prior to shaping.
By co-precipitation, the incorporation of a dispersed metals content into a conventional carrier material is attempted by enabling intimate contact between metals compounds and carrier material and thus enabling the metals to become dispersed through the carrier material before shaping. This contrasts with conventional impregnation techniques where only a small amount of metals deposition is possible since the shaped carrier is already formed and there are diffusional and space limitations for the metal ions or compounds to become dispersed through the catalyst support.
In U.S. Pat. No. 6,162,350 the use of other catalytic components with the bulk catalysts is envisaged. Thus cracking components such as ZSM-5, zeolite Y and amorphous cracking components may be composited with the bulk catalyst composition. Preferably this occurs after the composition is formed and alongside the incorporation of a binder material prior to shaping to form a cracking catalyst support in conventional manner.
Copending International Patent Application No. PCT/EP2004/050196, published as WO 2004/073859, discloses the preparation of a quasi-bulk metal catalyst composition in which binder materials are advantageously incorporated into the compositions during precipitation. Following formation, this material too can be composited with other components such as cracking components.
Thus where cracking components are to be incorporated into such bulk metal compositions the teaching is that this is preferred via mixing or co-mulling after the preparation of the bulk metal composition.
Coprecipitation of a zeolitic material and a Group VIb hydrogenation metal component to form a hydrocracking catalyst is disclosed in U.S. Pat. No. 3,853,747 which proposes the preparation of a hydrocracking catalyst having improved activity by combining a finely divided Group VIb (eg molybdenum) compound in substantially undissolved form with a crystalline aluminosilicate base in an aqueous medium having a pH below 6. A precipitated metals-containing zeolite results. The pH level is set at below 6 to ensure the insolubility of the metal compound and to promote precipitation without destruction of the zeolites crystalline structure. The aim of the preparation is to ensure that the metal is concentrated on the external surface of the zeolite, to avoid impregnation of the inner adsorption area, and to ensure that the zeolite crystal structure and acidity is not destroyed at any stage during the catalyst preparation by use of soluble molybdenum or tungsten compounds.
International Patent Specification No. WO 01/00753 considers the effect on middle distillate selectivity where an amount of hydrogenation metals is contained within the pores of a zeolite before formulation into a hydrocracking catalyst. Middle distillate selectivity gain is said to be achieved by incorporation of the hydrogenation metals into the zeolite pores, eg by impregnation of the zeolite with the metals prior to shaping the support. Broad ranges of from 0.1 to 10 wt % of Group VIII metal (on oxide basis) and from 0.1 to 10 wt % of Group VIb metal (on oxide basis) are disclosed, but the most preferred amounts incorporated are from 0 (sic) to 5 wt %, of Group VIII metal (on oxide basis)and from 0.1 to 3 wt %, of Group VIb metal (on oxide basis). A certain amount of hydrogenation metals will, however, inevitably be incorporated into the zeolitic pores during conventional catalyst preparation. We have found that in fact by conventional impregnation preparation techniques this preferred level of Group VIb metal, eg molybdenum, can commonly exist in zeolite Y based hydrocracking catalysts.
Generally the teaching of the art is that while it can be advantageous to incorporate hydrogenation metals into the pores of zeolitic materials, precipitation techniques have to be utilised carefully or risk the destruction of the zeolite crystal structure.